I can't help with exact answers but am also interested in the topic. I had formerly found the Soundtraxx document interesting for an overview. Note that it includes a not complete list of horn types by RR that might be helpful if you knew the timeframe of the locos and specific horn types (ignoring upgrades that might occur).
I've found some related info on Nathans and less useful info on Leslies, which your BN and prior RRs might have favored. I imagine you could find better info. All this might help narrow down which LokSound option to favor. I am pretty loose about that, often trying each of the horns and picking one (for starters) that sounds good with my speaker and might be in the range basis era of the prototype.
Here's a wild idea, a long shot. Maybe someone at BNSF would know the answer, since not terribly old locos, if you could get to the right person (in maintenance, engineering, etc). Maybe call one of the BNSF stores and ask who in the company you might talk with to get connected. Maybe worth a couple of calls.
Another idea: you might inquire of Matt Hermann at ESu LokSound (Pennsylvania) as he might know which of the 15 are typical for the loco type, to at least narrow it down. They have an email on the website or you might ask at the Yahoo LokSound user group.